mass transport in plants

Question 1

what type of cells are xylem

Answer 1

undifferentiated

Question 2

what adaptions do xylem cells have to carry out their function

Answer 2

• long, dead, cells which form hollow tubes - so water potential is not effected and water stays in the xylem
• end walls break down - water can form a continuous column
• cell walls are strengthened by lignin - makes xylem waterproof and helps withstand tension
• no cytoplasm - so they don’t obstruct flow of water
• many narrow xylem vessels - increases surface rea for adhesion

Question 3

define transpiration

Answer 3

the evaporation of water from the leaf through the stomata

Question 4

describe and explain how water moves through xylem vessels (cohesion tension theory)

Answer 4

• water evaporates from the leaf through stoma
• leads to a lower water potential in mesophyll cells then in the xylem creating a water potential gradient
• water moves from xylem into mesophyll cells by osmosis. creates tension and pulls water up the xylem
• water molecules form a continuous column through cohesion as they are held together by H bonds
• H bonding causes adhesion of water molecules to walls of xylem to cause xylem to be pulled inwards

Question 5

explain why transpiration is important

Answer 5

• provides leaves with water for photosynthesis
• provides water to make cells turgid
• provides water for hydrolysis reactions

Question 6

list the factors effecting rate of transpiration

Answer 6

• air movement
• temperature
• humidity
• light intensity

Question 7

what is the function of the phloem

Answer 7

transport organic substances in plants

Question 8

what are the features of phloem and their functions

Answer 8

• end walls perforated with holes - allows continuous flow if substances through sieve tube
• sieve tubes have no nucleus and little cytoplasm- allows unobstructed flow of solutes
• companion cells contain many organelles - carry out functions for sieve tubes

Question 9

what is translocation

Answer 9

movement of organic substances or assimilates to where they are needed in the plant

Question 10

what is a source

Answer 10

area of plant which produces assimilates

Question 11

what is a sink

Answer 11

where assimilates are sued or stored `

Question 12

decsribe the process of the mass flow hypothesis

Answer 12

• at source, active transport is used to load the sucrose made from photosynthesis from companion cells into the sieve tubes in the phloem
• this lowers water potential in phloem so water moves form xylem to sieve tubes by osmosis
• creates a higher hydrostatic pressure at the source and of the phloem so there is mass flow of solutes towards the sink
• qt sink, solutes are removed from phloem down conc. grad. where they are used up or stored as another molecules at the sink
• as solutes leave phloem, the water potential inside sieve tubes increases so water moves out of sieve tubes by osmosis
• therefore a pressure gradient is maintained from source to sink

Question 13

how does water evaporating out of the stoma affect the rate of transpiration

Answer 13

as water evaporates out, it accumulates as vapour around the stoma
- water potential around stoma increases and the water potential gradient decreases between inside and outside the leaf.
- transpiration rate decreases.

Question 14

explain why the faster the air movement around the stoma the higher the rate of transpiration

Answer 14

• movement of air around leaf decreases the water potential
• water potential gradient increases
• rate of transpiration increases
  -the faster the air movement, the greater the rate of transpiration

Question 15

explain how a rise in temperature increases the rate of transpiration

Answer 15

• increased temperature increases the kinetic energy of H2O molecules
• increases rate if evaporation
• increases rate of transpiration

Question 16

when the air outside the leaf has a high humidity, how does this affect the water potential gradient and the rate of transpiration

Answer 16

• the higher the humidity, the higher the water potential in the air
• so reduces water potential gradient between the leaf and the air
• so decreases rate of transpiration

Question 17

how does light intensity affect rate of transpiration

Answer 17

increased light intensity leads to increased rate of transpiration

Question 18

describe how a simple photometer may be used to measure the rate of water uptake and that this is used as a measure of transpiration

Answer 18

• leafy shoot is cut underwater to prevent air entering the xylem which would prevent water flow and is placed in a rubber tube
• the photometer is filled completely with water making sure there are no air bubbles
• potometer is removed from under water as all joint are sealed with waterproof jelly to prevent air leaking out which could produce an unreliable result
• an air bubble is introduced into the capillary tube
• as transpiration occurs, water moves through the capillary tube and into the the plant, and the bubble of air moves with it
• the distance moved over time is recorded and means is calculated of a number of repeats

Question 19

describe evidence of and against the mass flow hypothesis

Answer 19

FOR:
- ring of bark is removed and bulge forms above it. shows movement of sucrose can occur downward as sugars can’t move past area where bark has been cut
- aphid’s. out parts can pierce the phloem allowing to feed. if body of aphid is snapped off then phloem sap flows out quicker due to pressure gradient (higher pressure at leaves, lower pressure at sink)
- addition of metabolic inhibitors into the phloem stops translocation as they stop ATP production so active transport cannot occur so translocation stops

AGAINST:
- sugars have been shown to travel to all areas of the plant as sugars flow from source to sink according to hypothesis.